Apparatus and method for a current sensor

ABSTRACT

A current sensor may include a sense element disposed proximate a conductor. The current sensor may be configured to couple to a first magnetic field generated at the conductor when current flows in the conductor. An output electrically connected to the sense element can produce a signal that is representative of the flow of current in the conductor. The sense element may be oriented in a plane parallel to magnetic field lines of a second magnetic field generated by a load connected to the conductor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to commonly owned U.S. application Ser. No.14/618,343, filed Feb. 10, 2015.

TECHNICAL FIELD

The present disclosure generally relates to current sensors. Morespecifically, the disclosure is directed to devices, systems, andmethods related to current sensors using magnetic induction.

BACKGROUND

Wireless power transfer is an increasingly popular capability inportable electronic devices, such as mobile phones, computer tablets,etc. because such devices typically require long battery life and lowbattery weight. The ability to power an electronic device without theuse of wires provides a convenient solution for users of portableelectronic devices. Wireless power charging systems, for example, mayallow users to charge and/or power electronic devices without physical,electrical connections, thus reducing the number of components requiredfor operation of the electronic devices and simplifying the use of theelectronic device.

Wireless power transfer allows manufacturers to develop creativesolutions to problems due to having limited power sources in consumerelectronic devices. Wireless power transfer may reduce overall cost (forboth the user and the manufacturer) because conventional charginghardware such as power adapters and charging chords can be eliminated.There is flexibility in having different sizes and shapes in thecomponents (e.g., magnetic coil, charging plate, etc.) that make up awireless power transmitter and/or a wireless power receiver in terms ofindustrial design and support for a wide range of devices, from mobilehandheld devices to computer laptops.

SUMMARY

In some aspects of the present disclosure, a current sensor may includea sense element configured to couple to a first magnetic field generatedat a first location that can arise due a flow of current in theconductor. The current sensor may include an output that is electricallyconnected to the sense element to produce a signal that isrepresentative of the flow of current in the conductor. The senseelement may be oriented in a plane parallel to magnetic field lines of asecond magnetic field, different from the first magnetic field,generated by a load connected to the conductor at a second locationdifferent from the first location.

In some aspects, the sense element may comprise an electricallyconductive coil disposed on a substrate and positioned adjacent theconductor. The current sensor may further include a filter circuitcomprising the electrically conductive coil connected to a resistor anda capacitor. In some aspects, the filter circuit may be a bandpassfilter. In some aspects, the substrate that carries the electricallyconductive coil may be disposed perpendicular to a printed circuit board(PCB) that carries the conductor.

In some aspects, the load is a power transmitting element configured togenerate an external magnetic field for wireless power transfer. Theexternal magnetic field constitutes the second magnetic field. In someaspects, the sense element may include an electrically conductive coil.In some aspects, the current sensor may include filter comprising theelectrically conductive coil of the sense element. The filter may betuned to a resonant frequency of the power transmitting element.

In some aspects, the current sensor may further include a capacitiveshield disposed adjacent the sense element. The capacitive shield canprevent an electric field generated by the voltage in the conductor frombeing capacitively coupled to the sense element. In some aspects, thecapacitive shield may be a conductive lead disposed adjacent to both thesense element and the conductor. The conductive lead may have a freefirst end and a second end configured for a connection to groundpotential.

In some aspects, the sense element may include a first electricallyconductive coil disposed on a first plane and at least a secondelectrically conductive coil disposed on at least a second plane spacedapart from the first plane. The first electrically conductive coil maybe connected in series with the second electrically conductive coil. Insome aspects, the first electrically conductive coil may be a traceformed on a first layer of a multi-layer PCB and the second electricallyconductive coil may be a trace formed on a second layer of themulti-layer PCB. In some aspects, the first electrically conductive coilmay be disposed on one side of the conductor and the second electricallyconductive coil may be disposed on another side of the conductor.

In some aspects, the current sensor may include a first capacitiveshield configured to be disposed adjacent to both the first electricallyconductive coil and the conductor, and at least a second capacitiveshield configured to be disposed adjacent to both the secondelectrically conductive coil and the conductor.

In some aspects, the current sensor may include a filter circuitcomprising either or both the first electrically conductive coil and thesecond electrically conductive coil.

In some aspects, the sense element may include a first electricallyconductive coil and at least a second electrically conductive coil, thefirst and second coils both spaced apart from each other on the sameplane.

In some aspects according to the present disclosure, a method of sensinga flow of current in a conductor may include generating a sensed signalby magnetically coupling to a first magnetic field arising from theconductor due a flow of current in the conductor and generating anoutput signal from the sensed signal. The magnetic coupling may occur ina plane parallel to field lines of a second magnetic field generated bythe flow of current through a load electrically connected to theconductor.

In some aspects, magnetically coupling to the first magnetic field mayinclude inducing a current, using the first magnetic field, in a coil ofelectrically conductive material disposed adjacent the conductor andaligned in the plane parallel to field lines of the second magneticfield.

In some aspects, the method may further include filtering the sensedsignal to attenuate frequency components in the sensed signal determinedby a filter circuit that includes the coil of electrically conductivematerial.

In some aspects, the load may be a power transmitting element configuredto generate an external magnetic field for wireless power transfer,wherein the external magnetic field constitutes the second magneticfield.

In some aspects, the method may further include shielding the sensedsignal from an electric field generated in the conductor so that thegenerated output signal is substantially free of influence from theelectric field.

In some aspects, magnetically coupling to the first magnetic field mayinclude coupling the first magnetic field to a first coil ofelectrically conductive material disposed adjacent the conductor andcoupling the first magnetic field to a second coil of electricallyconductive material disposed adjacent the conductor.

In some aspects of the present disclosure, an apparatus for sensing aflow of current in a conductor may include means for magneticallycoupling to a first magnetic field arising from the conductor due to aflow of current in the conductor to generate a sensed signal and meansfor generating an output signal from the sensed signal. The means formagnetically coupling to a first magnetic field may be aligned in aplane parallel to field lines of a second magnetic field generated bythe flow of current through a load electrically connected to theconductor.

In some aspects, the apparatus may include means for filtering thesensed signal including the means for magnetically coupling to a firstmagnetic field. The means for magnetically coupling to a first magneticfield may be a coil of electrically conductive material. The means forfiltering may be a bandpass filter.

In some aspects of the present disclosure, an apparatus for wirelesslytransmitting charging power to a receiver device may include a transmitcoil, a driver circuit electrically coupled to the transmit coil via aconductor, and a current sensor configured to sense a flow of current inthe conductor. The driver circuit may be configured to drive thetransmit coil with an alternating current via the conductor. The currentsensor may include a sense coil configured to couple to a first magneticfield generated by the alternating current in the conductor to produce asignal that is indicative of the flow of current in the conductor. Thetransmit coil may be configured to generate a second magnetic field forwirelessly transmitting charging power to the receiver device inresponse to being driven by the alternating current. The sense coil maybe oriented in a plane parallel to field lines of the second magneticfield.

In some aspects, the apparatus may include a filter circuit comprisingthe sense coil and a resistor and capacitor electrically connected tothe sense coil. The filter may be a bandpass filter.

In some aspects, the current sensor may include a capacitive shielddisposed adjacent to the sense coil. The capacitive shield can preventan electric field generated in the conductor from being capacitivelycoupled to the sense coil, wherein the capacitive shield comprises asecond conductor disposed between the conductor and the sense coil.

The following detailed description and accompanying drawings provide abetter understanding of the nature and advantages of the presentdisclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

With respect to the discussion to follow and in particular to thedrawings, it is stressed that the particulars shown represent examplesfor purposes of illustrative discussion, and are presented in the causeof providing a description of principles and conceptual aspects of thepresent disclosure. In this regard, no attempt is made to showimplementation details beyond what is needed for a fundamentalunderstanding of the present disclosure. The discussion to follow, inconjunction with the drawings, makes apparent to those of skill in theart how embodiments in accordance with the present disclosure may bepracticed. In the accompanying drawings:

FIG. 1 is a functional block diagram of a wireless power transfer systemin accordance with an illustrative embodiment.

FIG. 2 is a functional block diagram of a wireless power transfer systemin accordance with an illustrative embodiment.

FIG. 3 is a schematic diagram of a portion of transmit circuitry orreceive circuitry of FIG. 2 including a power transmitting or receivingelement in accordance with an illustrative embodiment.

FIGS. 4A and 4B represent illustrative configurations that embody acurrent sensor in accordance with the present disclosure.

FIGS. 5A, 5B, and 5C illustrate aspects of a current sensor inaccordance with the present disclosure.

FIG. 6 illustrates details of a current sensor in accordance with thepresent disclosure.

FIGS. 6A and 6B illustrate additional embodiments of a current sensor.

FIG. 7 illustrates a configuration of a current sensor in accordancewith the present disclosure.

FIG. 8 illustrates a filter configured in accordance with the presentdisclosure.

FIGS. 9 and 9A illustrate additional embodiments of a current sensor.

FIGS. 10A, 10B, 10C, and 10D show illustrative configurations of currentsensors in accordance with the present disclosure.

DETAILED DESCRIPTION

Wireless power transfer may refer to transferring any form of energyassociated with electric fields, magnetic fields, electromagneticfields, or otherwise from a transmitter to a receiver without the use ofphysical electrical conductors (e.g., power may be transferred throughfree space). The power output into a wireless field (e.g., a magneticfield or an electromagnetic field) may be received, captured by, orcoupled by a “power receiving element” to achieve power transfer.

FIG. 1 is a functional block diagram of a wireless power transfer system100, in accordance with an illustrative embodiment. Input power 102 maybe provided to a transmitter 104 from a power source (not shown in thisfigure) to generate a wireless (e.g., magnetic or electromagnetic) field105 for performing energy transfer. A receiver 108 may couple to thewireless field 105 and generate output power 110 for storing orconsumption by a device (not shown in this figure) coupled to the outputpower 110. The transmitter 104 and the receiver 108 may be separated bya distance 112. The transmitter 104 may include a power transmittingelement 114 for transmitting/coupling energy to the receiver 108. Thereceiver 108 may include a power receiving element 118 for receiving orcapturing/coupling energy transmitted from the transmitter 104.

In one illustrative embodiment, the transmitter 104 and the receiver 108may be configured according to a mutual resonant relationship. When theresonant frequency of the receiver 108 and the resonant frequency of thetransmitter 104 are substantially the same or very close, transmissionlosses between the transmitter 104 and the receiver 108 are reduced. Assuch, wireless power transfer may be provided over larger distances.Resonant inductive coupling techniques may thus allow for improvedefficiency and power transfer over various distances and with a varietyof inductive power transmitting and receiving element configurations.

In certain embodiments, the wireless field 105 may correspond to the“near field” of the transmitter 104. The near-field may correspond to aregion in which there are strong reactive fields resulting from thecurrents and charges in the power transmitting element 114 thatminimally radiate power away from the power transmitting element 114.The near-field may correspond to a region that is within about onewavelength (or a fraction thereof) of the power transmitting element114.

In certain embodiments, efficient energy transfer may occur by couplinga large portion of the energy in the wireless field 105 to the powerreceiving element 118 rather than propagating most of the energy in anelectromagnetic wave to the far field.

In certain implementations, the transmitter 104 may output a timevarying magnetic (or electromagnetic) field with a frequencycorresponding to the resonant frequency of the power transmittingelement 114. When the receiver 108 is within the wireless field 105, thetime varying magnetic (or electromagnetic) field may induce a current inthe power receiving element 118. As described above, if the powerreceiving element 118 is configured as a resonant circuit to resonate atthe frequency of the power transmitting element 114, energy may beefficiently transferred. An alternating current (AC) signal induced inthe power receiving element 118 may be rectified to produce a directcurrent (DC) signal that may be provided to charge or to power a load.

FIG. 2 is a functional block diagram of a wireless power transfer system200, in accordance with another illustrative embodiment. The system 200may include a transmitter 204 and a receiver 208. The transmitter 204(also referred to herein as power transfer unit, PTU) may includetransmit circuitry 206 that may include an oscillator 222, a drivercircuit 224, and a front-end circuit 226. The oscillator 222 may beconfigured to generate a signal at a desired frequency that may adjustin response to a frequency control signal 223. The oscillator 222 mayprovide the oscillator signal to the driver circuit 224. The drivercircuit 224 may be configured to drive the power transmitting element214 at, for example, a resonant frequency of the power transmittingelement 214 based on an input voltage signal (VD) 225. The drivercircuit 224 may be a switching amplifier configured to receive a squarewave from the oscillator 222 and output a sine wave.

The front-end circuit 226 may include a filter circuit configured tofilter out harmonics or other unwanted frequencies. The front-endcircuit 226 may include a matching circuit configured to match theimpedance of the transmitter 204 to the impedance of the powertransmitting element 214. As will explained in more detail below, thefront-end circuit 226 may include a tuning circuit to create a resonantcircuit with the power transmitting element 214. As a result of drivingthe power transmitting element 214, the power transmitting element 214may generate a wireless field 205 to wirelessly output power at a levelsufficient for charging a battery 236, or otherwise powering a load.

The transmitter 204 may further include a controller 240 operablycoupled to the transmit circuitry 206 configured to control one oraspects of the transmit circuitry 206 or accomplish other operationsrelevant to managing the transfer of power. The controller 240 may be amicro-controller or a processor. The controller 240 may be implementedas an application-specific integrated circuit (ASIC). The controller 240may be operably connected, directly or indirectly, to each component ofthe transmit circuitry 206. The controller 240 may be further configuredto receive information from each of the components of the transmitcircuitry 206 and perform calculations based on the receivedinformation. The controller 240 may be configured to generate controlsignals (e.g., signal 223) for each of the components that may adjustthe operation of that component. As such, the controller 240 may beconfigured to adjust or manage the power transfer based on a result ofthe operations performed by it. The transmitter 204 may further includea memory (not shown) configured to store data, for example, such asinstructions for causing the controller 240 to perform particularfunctions, such as those related to management of wireless powertransfer.

The receiver 208 (also referred to herein as power receiving unit, PRU)may include receive circuitry 210 that may include a front-end circuit232 and a rectifier circuit 234. The front-end circuit 232 may includematching circuitry configured to match the impedance of the receivecircuitry 210 to the impedance of the power receiving element 218. Aswill be explained below, the front-end circuit 232 may further include atuning circuit to create a resonant circuit with the power receivingelement 218. The rectifier circuit 234 may generate a DC power outputfrom an AC power input to charge the battery 236, as shown in FIG. 2.The receiver 208 and the transmitter 204 may additionally communicate ona separate communication channel 219 (e.g., Bluetooth, Zigbee, cellular,etc.). The receiver 208 and the transmitter 204 may alternativelycommunicate via in-band signaling using characteristics of the wirelessfield 205.

The receiver 208 may be configured to determine whether an amount ofpower transmitted by the transmitter 204 and received by the receiver208 is appropriate for charging the battery 236. In certain embodiments,the transmitter 204 may be configured to generate a predominantlynon-radiative field with a direct field coupling coefficient (k) forproviding energy transfer. Receiver 208 may directly couple to thewireless field 205 and may generate an output power for storing orconsumption by a battery (or load) 236 coupled to the output or receivecircuitry 210.

The receiver 208 may further include a controller 250 configuredsimilarly to the transmit controller 240 as described above for managingone or more aspects of the wireless power receiver 208. The receiver 208may further include a memory (not shown) configured to store data, forexample, such as instructions for causing the controller 250 to performparticular functions, such as those related to management of wirelesspower transfer.

As discussed above, transmitter 204 and receiver 208 may be separated bya distance and may be configured according to a mutual resonantrelationship to minimize transmission losses between the transmitter 204and the receiver 208.

FIG. 3 is a schematic diagram of a portion of the transmit circuitry 206or the receive circuitry 210 of FIG. 2, in accordance with illustrativeembodiments. As illustrated in FIG. 3, transmit or receive circuitry 350may include a power transmitting or receiving element 352 and a tuningcircuit 360. The power transmitting or receiving element 352 may also bereferred to or be configured as an antenna or a “loop” antenna. The term“antenna” generally refers to a component that may wirelessly output orreceive energy for coupling to another “antenna.” The power transmittingor receiving element 352 may also be referred to herein or be configuredas a “magnetic” antenna, or an induction coil, a resonator, or a portionof a resonator. The power transmitting or receiving element 352 may alsobe referred to as a coil or resonator of a type that is configured towirelessly output or receive power. As used herein, the powertransmitting or receiving element 352 is an example of a “power transfercomponent” of a type that is configured to wirelessly output and/orreceive power. The power transmitting or receiving element 352 mayinclude an air core or a physical core such as a ferrite core (not shownin this figure).

When the power transmitting or receiving element 352 is configured as aresonant circuit or resonator with tuning circuit 360, the resonantfrequency of the power transmitting or receiving element 352 may bebased on the inductance and capacitance. Inductance may be simply theinductance created by a coil or other inductor forming the powertransmitting or receiving element 352. Capacitance (e.g., a capacitor)may be provided by the tuning circuit 360 to create a resonant structureat a desired resonant frequency. As a non limiting example, the tuningcircuit 360 may comprise a capacitor 354 and a capacitor 356 may beadded to the transmit and/or receive circuitry 350 to create a resonantcircuit.

The tuning circuit 360 may include other components to form a resonantcircuit with the power transmitting or receiving element 352. As anothernon limiting example, the tuning circuit 360 may include a capacitor(not shown) placed in parallel between the two terminals of thecircuitry 350. Still other designs are possible. In some embodiments,the tuning circuit in the front-end circuit 226 may have the same design(e.g., 360) as the tuning circuit in front-end circuit 232. In otherembodiments, the front-end circuit 226 may use a tuning circuit designdifferent than in the front-end circuit 232.

For power transmitting elements, the signal 358, with a frequency thatsubstantially corresponds to the resonant frequency of the powertransmitting or receiving element 352, may be an input to the powertransmitting or receiving element 352. For power receiving elements, thesignal 358, with a frequency that substantially corresponds to theresonant frequency of the power transmitting or receiving element 352,may be an output from the power transmitting or receiving element 352.Although aspects disclosed herein may be generally directed to resonantwireless power transfer, persons of ordinary skill will appreciate thataspects disclosed herein may be used in non-resonant implementations forwireless power transfer.

Accurate measurement of the current injected into the power transmittingelement 214 shown in FIG. 2 (e.g., transmit coil) may be used tomaintain proper levels of electromagnetic (EM) radiation that radiateinto the environment, measure power transfer characteristics, and thelike. Some solutions may be based on measuring the voltage drop acrosstwo series capacitors (e.g., using a differential amplifier), onecapacitor attached to each of the coil leads of the transmit coil.Measuring the voltage directly in this way can create technicalchallenges relating to the design of the differential voltage buffer andamplifier circuits that comprise a differential amplifier because bothcoil leads are at a high voltage. In addition, this process for makingmeasurements can be complex, requiring the measurement of the voltageacross the power transmitting element 214 behind the series capacitorsand then measuring the voltage after the series capacitors, along withfast switching of voltages that feed into low pass filters, and so on.The circuitry may require costly components to implement; and theprocess of taking measurements can create a good amount ofelectromagnetic interference (EMI) due to switching noise which can beinjected into the power transmitting element 214.

Referring to FIGS. 4A and 4B, the discussion will now turn to adescription of current sensors in accordance with the present disclosurethat may be used to make measurements of current injected into a load.FIG. 4A shows circuitry comprising a power amplifier 40 connected to aload 44 via a current-carrying conductor 42. A current sensor 402 inaccordance with the present disclosure may be configured to sense theflow of current in the current-carrying conductor 42 and produce asignal that is representative of the flow of current in thecurrent-carrying conductor 42.

Merely as an example to illustrate a usage case, the current sensor 402may be incorporated in the wireless power transfer system 200 shown inFIG. 2. In this example, the power amplifier 40 may correspond to thedriver circuit 224 in transmitter 204, and the load 44 may correspond tothe power transmitting element 214 (e.g., transmit coil). The currentsensor 402 may detect load changes in the power transmitting element 214during wireless power transfer as a consequence of variations in theamount of power that is being coupled to a receiver 208 (e.g., PRU) viathe magnetic field generated by the power transmitting element 214. Forexample, variations in power coupling may arise from the amount of powera PRU draws, the number of PRUs engaged in wireless power transfer withthe transmitter 204, and so on. The current-carrying conductor 42 maycorrespond to a connection (e.g., a wire not shown) that providescurrent from the driver circuit 224 to the power transmitting element214. It will be appreciated, of course, that a current sensor 402 inaccordance with the present disclosure may be readily adapted for use inother circuits.

The current sensor 402 may include connections 404 and 406 to providepoints of connection for the current-carrying conductor 42. The currentsensor 402 may include outputs 408 to output a sensed signal in responseto the flow of current in conductor 42. The outputs 408 may be connectedto a means for generating an output signal from the sensed signal, suchas amplifier 48, for example, to produce an output signal V_(out) thatrepresents or is otherwise indicative of the flow of current in thecurrent-carrying conductor 42. In some embodiments, the output ofamplifier 48 may be a current signal that represents the flow of currentin the current-carrying conductor 42. In other embodiments, such asshown in FIG. 4A, the output of amplifier 48 may be an output voltageV_(out) that represents the flow of current in the current-carryingconductor 42. In some embodiments, the output of amplifier 48 may beused as a feedback signal to control the flow of current out of thepower amplifier 40. In other embodiments, the output of amplifier 48 maybe used to monitor the operating conditions of the system. For example,in the context of the wireless power transfer system 200 shown in FIG.2, in some embodiments, the current sensor 402 may be used to detect anoverload condition. In other embodiments, the current sensor 402 may beused to detect placement of a PRU on the charging surface of thetransmitter 204, and so on.

The power amplifier 40 in FIG. 4A represents an example of asingle-ended output. Referring to FIG. 4B, in some embodiments, a poweramplifier 40 a may have a differential output configured to providepower using two current-carrying conductors 42 a and 42 b. Accordingly,a current sensor 412 in accordance with some embodiments of the presentdisclosure may be configured to provide current sensing on multiplecurrent-carrying conductors (e.g., 42 a, 42 b). In some embodiments, forexample, the current sensor 412 may include connections 404 a, 404 b and406 a, 406 b to provide points of connection for the current-carryingconductors 42 a, 42 b.

FIG. 5A shows a current sensor 402 configured in accordance with thepresent disclosure. The current sensor 402 may be disposed on a circuitboard 502. In some embodiments, for example, the circuit board 502 maybe an electronic component in a power transmitting unit (e.g.,transmitter 204, FIG. 2) of a wireless power transfer system (e.g., FIG.2). The current sensor 402 may be disposed or otherwise positionedadjacent or near a conductor 542 supported on the circuit board 502. Insome embodiments, the conductor 542 may be a wire affixed to the circuitboard 502. In other embodiments, the conductor 542 may be a trace formedon the circuit board 502, and so on.

The conductor 542 may be connected to a transmit coil 544 of thewireless power transfer system. In some embodiments, the current sensor402 may be placed adjacent the conductor 542 at the high side of a powersource (e.g., power amplifier 40, FIG. 4A). For example, the conductor542 may be connected between the output of the power source and thetransmit coil 544. In other embodiments, the current sensor 402 may bepositioned at the low side of conductor 542. For example, the conductor542 may be connected between the transmit coil 544 and ground. Since, insome embodiments, the voltage in the conductor 542 can be tens of volts,placing the current sensor 402 on the low-side of conductor 542 canreduce the voltage by a factor of two to three orders of magnitude, thusreducing the induced voltage in the current sensor 402.

In accordance with the present disclosure, the current sensor 402 may beoriented relative to a magnetic field 546, which can be generated by thetransmit coil 544 during a wireless power transfer operation, such thatthe field lines intersect a minimal cross-sectional area of the currentsensor 402. In some embodiments, for example, the current sensor 402 maybe oriented in a plane parallel to some field lines (flux) of themagnetic field 546. In the particular embodiment shown in FIG. 5A, forexample, the transmit coil 544 is oriented in the same plane as thecircuit board 502. Accordingly, the current sensor 402 is disposedperpendicularly relative to the circuit board 502, since the magneticfield 542 would be perpendicular to the transmit coil 544. In otherembodiments, the transmit coil 544 may lie at a non-perpendicular anglerelative to the circuit board 502. The current sensor 402, accordingly,would be attached to the circuit board 502 at a suitable angle so as tobe oriented in a plane parallel to field lines of the magnetic field 546from transmit coil 544.

The plan view in FIG. 5B and the top view in FIG. 5C further illustratethe relative orientation between the current sensor 402 and magneticfield 546 in accordance with the present disclosure. An XYZ coordinateis used to establish a reference. In some embodiments for example,without loss of generality, circuit board 502 and transmit coil 544 liein the XY plane. Accordingly, the current sensor 402 is oriented in theXZ plane. The plan view in FIG. 5B shows certain field lines 546 a ofmagnetic field 546 that lie the XZ plane, and the orientation of currentsensor 402 is also in a plane parallel to those field lines 546 a. Theorientation of current sensor 402 relative to field lines 546 a isfurther illustrated in the top view shown in FIG. 5C.

FIG. 6 shows details of current sensor 402 in accordance with thepresent disclosure, along with some circuit elements shown in FIG. 4Afor context. The inset represents a view taken along view line A-A. Theconductor 542 may connect a power supply (e.g., power amplifier 40, FIG.4A) to the transmit coil 544. As can be seen in the inset, the conductor542 and the current sensor 402 may be disposed on the circuit board 502,as described above. The XYZ coordinate reference established in FIG. 5Aillustrates the relative orientation of the current sensor 402 and theconductor 542 on the circuit board 502.

In some embodiments, the current sensor 402 may comprise a sense element604 disposed on a plane, for example, as defined by a substrate 632. Thesense element 604 may comprise a coil 612 (or loop) of electricallyconductive material. The substrate 632 may be a printed circuit board(PCB). The coil 612 may be a trace or a plurality of trace segmentsformed on the substrate 632. The conductive material used for coil 612may be copper or any suitable electrically conductive material. Theconductive material may be formed or otherwise deposited on thesubstrate 632 using any of a number of known techniques.

FIG. 6 depicts the coil 612 formed on a first face of the substrate 632.In some embodiments, the coil 612 may have one or more turns. The outerend 612 b of the coil 612 may terminate at a conductive pad B on thesubstrate 632. The inner end 612 a of the coil 612 may terminate at aconductive pad A on the substrate 632 by way of a return path thatcomprises vias 614 and 618 formed through the substrate 632 and a trace616 formed on a second face of the substrate 632 that connects via 614to via 618. A trace may connect the via 618 to pad A. An amplifier(e.g., 48, FIG. 4A) may be connected to the pads A, B.

In accordance with the present disclosure, the current sensor 402 mayfurther comprise a capacitive shield 622 disposed adjacent to the senseelement 604, and to the conductor 542, for example, by virtue of thecurrent sensor 402 being positioned near the conductor 542. In someembodiments, the capacitive shield 622 may comprise a conductive trace(lead) formed on the substrate 632. One end 622 a of the capacitiveshield 622 may be “free,” or not otherwise connected. Another end 622 bof the capacitive shield 622 may be electrically connected to aconductive pad C via a trace 624. In some embodiments, the pad C may beconnected to ground potential. In other embodiments, the pad B and thepad C may be connected to a common voltage reference. This aspect of thepresent disclosure will be discussed below.

In operation, with reference to FIGS. 5A and 6, when an AC drive currentis provided to drive the transmit coil 544, the flow of the currentthrough conductor 542 can generate a magnetic field 548 (inset, FIG. 6)at a first location in the conductor 542 near the current sensor 402.The magnetic field 548, being in the Y-Z plane is generally orthogonalto the magnetic field 546 (e.g., FIG. 5B) which lies in the X-Z plane.The sense element 604 component of the current sensor 402 can serve as ameans for coupling to the magnetic field 548, which in turn can resultin an induced flow of current (sensed signal) in the sense element 604.The sensed signal generated in the sense element 604 can be provided atends 612 a, 612 b and amplified by amplifier 48 to produce an outputsignal V_(out) that is indicative of the amount of current flowingthrough the conductor 542.

As explained above, the current sensor 402 is based on magnetic couplingbetween the conductor 542 carrying the current and the coils 612(measurement loops) that comprise the sense element 604. Accordingly, anincrease in the coupling between conductor 542 and sense element 604 canimprove the signal to noise ratio (SNR) of the sensed signal. At thesame time, however, a reduction of the magnetic coupling between senseelement 604 and any other sources of magnetic fields may be preferablein order to avoid inaccuracies in the sense signal.

In some embodiments, the transmit coil 544 may be physically close tothe circuit board 502 that carries the current sensor 542. As a result,current sensor 402 can be exposed to the magnetic fields 546 generatedby the transmit coil 544; e.g., during a wireless power transferoperation. On the one hand, since such externally generated magneticfields 546 are proportional to the current, the resulting inducedcurrent flow (sensed signal) in the sense element 604 of the currentsensor 402 may improve the SNR. On the other hand, the magnetic field546 generated by the transmit coil 544 can be affected by the powerreceiving devices and other devices in the vicinity of the magneticfield. Accordingly, such variations in the magnetic field 546 can be asource of error for the current sensor 402. The error can be pronouncedif the magnetic field 546 varies (e.g., due to varying load conditionsat the receiver side) but the current flowing in conductor 542 isconstant. In other words, variations in the magnetic field 546 canproduce variations in the output signal V_(out) even though current flowin conductor 542 is constant. Since the current sensor 402 may be usedto provide feedback to adjust the magnetic field 546 or to detectforeign objects in the magnetic field 546, it may be beneficial toensure that the magnetic field 546 does not interfere with the sensedsignal.

In accordance with the present disclosure, as illustrated in FIGS. 5A-5Cfor example, magnetic shielding of the magnetic field 546 can beachieved by orienting the sense element 604 in parallel to the flux thatis generated by the transmit coil 544. In some implementations, forexample, the circuit board 502 is in the same plane as transmit coil544. Accordingly, the sense element 604 in such an implementation isoriented perpendicular to the circuit board 502.

It can be appreciated that orienting the sense element 604 relative tothe transmit coil 544 in accordance with the present disclosure canminimize the cross-sectional area of the sense element 604 that isintersected by the field lines of the magnetic field 546. Minimizing theintersected cross-section area results in minimizing the induced currenteffect of the magnetic field 546 generated by the transmit coil 544, andhence the effect of variations in the magnetic field 546 on the sensedsignal in the sense element 604.

Still referring to FIGS. 5A and 6, as explained above when current flowsthrough the conductor 542, a magnetic field 548 (see inset of FIG. 6)may arise from the conductor 542. The sense element 604, being in thevicinity of the conductor 542, may magnetically couple to the magneticfield 548. The area in the vicinity of the sense element 604 and theconductor 542 may be referred to as the sensing area. A voltage may beinduced in the sense element 604 that results from magnetically couplingto the magnetic field 548. The induced voltage may be amplified byamplifier 48 to generate an output voltage V_(out) representative of thecurrent flowing in the conductor 542.

The proximity of conductor 542 to the sense element 604 can create acapacitor. If the transmit coil 544 is driven by a high voltage at highfrequency, the capacitive coupling between conductor 542 and senseelement 604 may be significant even though the capacitance may be small.For example, in some embodiments, the transmit coil 544 may be driven bya 6.78 MHz signal on the order of tens of volts. As a result, capacitivecoupling of an electric field generated due to the voltage potential ofthe conductor 542 to the sense element 604 can be significant. Theenergy that can be coupled to the sense element 604 can create an errorin the generated output voltage V_(out).

In accordance with the present disclosure, the capacitive shield 622between conductor 542 and sense element 604 can provide electric fieldisolation. The capacitive shield 622 can isolate the electric field fromthe sense element 604 by capacitively coupling the electric field toground potential, thus shielding the output voltage V_(out) from theinfluence of the electric field generated by the conductor 542.

FIG. 6A shows a current sensor 402′ in accordance with some embodimentsof the present disclosure. In some embodiments, the current sensor 402′may comprise a sense element 604 comprising a first coil (or loop) ofconductive material 612-1 disposed on a first plane (e.g., as defined bya substrate 632-1) and a second coil of conductive material 612-2disposed on a second plane (e.g., as defined by a substrate 632-2).

In some embodiments, the substrates 632-1, 632-2 may be layers in amultilayer PCB 632′. The coils 612-1, 612-2 may be traces formed onrespective layers of the PCB 632′. The conductive material used to formcoils 612-1, 612-2 may be copper or any suitable material. The tracesmay be formed on the substrates 632-1, 632-2 using any of a number ofknown techniques.

In some embodiments, the coils 612-1, 612-2 may be connected in series,as shown in FIG. 6A for example. The outer end 612-1 b of the coil 612-1may terminate at a conductive pad B on the substrate 632-1. A via 614 bmay serve to connect the inner end 612-1 a of coil 612-1 on substrate632-1 to the inner end 612-2 a of coil 612-2 on substrate 632-2. A via614 c may connect the outer end 612-2 b of coil 612-2 on substrate 632-2to a conductive pad A on substrate 632-1.

In accordance with the present disclosure, the current sensor 402′ mayfurther comprise a first capacitive shield 622-1 disposed adjacent toboth the coil 612-1 of sense element 604 and the conductor 542, and asecond capacitive shield 622-2 disposed adjacent to the coil 612-2 ofsense element 604 and to the conductor 542 by virtue of the currentsensor 402′ being placed near the conductor 542. In some embodiments,the first capacitive shield 622-1 may comprise a conductive trace (lead)formed on substrate 632-1 and likewise the second capacitive shield622-2 may comprise a conductive trace (lead) formed on substrate 632-2.

In accordance with the present disclosure, the capacitive shields 622-1,622-2 may be connected together so that each capacitive shield has afree end and a grounded end, so that the capacitive shields do not forma closed loop. FIG. 6A, for example, shows a connection configuration inaccordance with some embodiments. One end 622-1 a of the capacitiveshield 622-1 may be “free,” or not otherwise connected. Another end622-1 b of the capacitive shield 622-1 may connect to a conductive padC, for example, via a trace 624. Likewise, one end 622-2 a of thecapacitive shield 622-2 may be “free,” or not otherwise connected.Another end 622-2b of the capacitive shield 622-2 may connect to aconductive pad C; for example, a via 614 a may connect end 622-2 b toend 622-1 a. In some embodiments, the pad C may be connected to groundpotential. In other embodiments, the pad B and the pad C may beconnected to a common voltage reference.

FIG. 6B shows a current sensor 402″ in accordance with some embodimentsof the present disclosure. In the configuration shown in FIG. 6B, thecapacitive shields 622-1, 622-2 may be connected in end-to-end fashionto form a continuous trace. For example, one end 622-2 b of capacitiveshield 622-2 may be the free end. The other end 622-2 a of capacitiveshield 622-2 may connect to one end 622-1 a of capacitive shield 622-1,for example, using via 614 a. The other end 622-1 b of capacitive shield622-1 may connect to pad C, for example, using trace 624. One ofordinary skill will appreciate that still other connectionconfigurations in accordance with the present disclosure may bepossible.

One of ordinary skill will appreciate that in accordance with thepresent disclosure, the sense element 604 in FIG. 6A or FIG. 6B maycomprise coils provided on respective additional layers of themulti-layer PCB 632′ in addition to coils 612-1, 612-2. In someembodiments, for example, the PCB 632′ may be an N-layer PCB supportinga sense element 604 comprising N coils, one coil in each layer.Accompanying each additional coil may be a capacitive shield (tracelead) disposed adjacent to the coil on the same layer (e.g., co-planarwith the coil).

FIG. 7 shows a current sensor 702 in accordance with some embodiments ofthe present disclosure. In some embodiments, a current sensor 702disposed on circuit board 502 may have a dual structure to sense thecurrent flow in conductor 542. The current sensor 702 may comprise afirst sensor component 702 a and a second sensor component 702 b spacedapart from the first sensor component 702 a. The current sensor 702 maystraddle the conductor 542 so that the first sensor component 702 a lieson one side of the conductor 542 and the second sensor component 702 blies in opposition to the first sensor component 702 a on the other sideof the conductor 542. The sense element (e.g., 604, FIG. 6) in the firstsensor component 702 a may comprise a coil (e.g., 612, FIG. 6) that iswound in the same direction, either clockwise or counterclockwise, asthe winding of a coil that comprises a sense element in the secondsensor component 702 b. The respective coils in the first and secondsensor components 702 a, 702 b may be connected together to sum thecurrent flow in the respective coils. This dual structure can provideincreased current sensing sensitivity as compared to the singlestructure current sensor 402 shown in FIG. 6.

FIG. 8 illustrates a means for filtering 800 the sensed signal inaccordance with the present disclosure. Referring first for a moment toFIG. 6A, measuring current flow in the conductor 542 may involvedetermining the root mean square (RMS) of the waveform of the sensedsignal induced in the sense element 612. The RMS may be determined bymeasuring the peaks in the sensed signal and determining the phase angleby detecting the zero crossings. This approach, however, has aconstraint in that the waveform of the sensed signal should not have anyharmonics.

The source of the harmonics can arise in the power amplifier (e.g., 40,FIG. 4A) that provides the current in the conductor 542. In someimplementations, the power amplifier may be a non-linear amplifier. Forexample, the high power output requirements of a wireless power systemand cost constraints may dictate a non-linear design. The current andvoltage produced by a non-linear power amplifier typically havesubstantial harmonic content.

In accordance with the present disclosure, a bandpass filter may be usedon the sensed signal to filter out the higher and lower harmonics at theoutput 408 of the current sensor 402, for current measurements. FIG. 8shows a means for filtering 800 the sensed signal in accordance withsome embodiments of the present disclosure. As described above, thesense element 604 (FIG. 6) comprises a coil 612 (or loop) ofelectrically conductive material. The coil 612, therefore, has aninductance L. The coil 612 can be used to create a bandpass filter 800using a resistor R and capacitor C. For example, resistor R may beconnected between pad A (FIG. 6) on the current sensor 402 and an inputto the amplifier 48. Capacitor C may be connected between pad B andanother input to the amplifier 48. The value of C may be determinedbased on the inductance L of the coil 612 and an operating frequency ofthe power supply. For example, in some implementations, the operatingfrequency may be the resonant frequency of the transmit coil 544 in thewireless power transfer system. The resistor R may be a small resistorthat can be added to detune the bandpass filter 800 and widen thebandpass frequencies in order to reduce sensitivity to actual componentvalues.

For voltage measurements, a capacitive voltage divider (not shown) maybe used. To achieve the desired harmonic rejection, for example, a7^(th) order, 4-stage Butterworth filter (not shown) may be used. Itwill be appreciated that any suitable bandpass filter design may beused.

FIG. 9 shows a current sensor 902 in accordance with some embodiments ofthe present disclosure. In some embodiments, the current sensor 902 maycomprise a sense element 904 disposed on a plane, for example, asdefined by substrate 932. The sense element 904 may comprise a firstcoil of conductive material 912-1 and a second coil of conductivematerial 912-2. The first and second coils 912-1, 912-2 may be connectedin series. For example, vias may be used to route traces on an oppositeface of the substrate 932 in order to connect the first and second coils912-1, 912-2 in series.

In some embodiments, the first and second coils 912-1, 912-2 thatcomprise sense element 904 may be substantially co-planar on thesubstrate 932 and in opposed relation to each other. In accordance withthe present disclosure, the sense element 904 may be oriented relativeto the field lines of a magnetic field generated by the transmit coil544 so as to minimize the area of intersection between the coils 912-1,912-2 and the field lines. In some embodiments, for example, the senseelement 904 may be oriented in a plane parallel to field lines of themagnetic field generated by the transmit coil 544.

The conductor 542 that provides drive current to the transmit coil 544from a power source (e.g., power amplifier 40, FIG. 4A) may have asegment 542-1 that runs along the surface of substrate 932. Theconductor segment 542-1 may run between the coils 912-1, 912-2. When acurrent flows through conductor 542 and hence conductor segment 542-1(e.g., during wireless power transfer), a magnetic field can begenerated. The magnetic field, in turn, can induce a flow of current inboth coils 912-1, 912-2, which can then be amplified (e.g., usingamplifier 48) to produce an output signal indicative of the flow ofcurrent in conductor 542.

In accordance with the present disclosure, the current sensor 902 mayfurther comprise a first capacitive shield 922-1 disposed adjacent toboth the first coil 912-1 and the conductor segment 542-1, and a secondcapacitive shield 922-2 disposed adjacent to both the second coil 912-2and the conductor segment 542-1. In some embodiments, the first andsecond capacitive shields 922-1, 922-2 may comprise conductive traces(leads) formed on the substrate 932. One end of respective first andsecond capacitive shields 922-1, 922-2 may be “free,” or not otherwiseconnected. Another end of respective first and second capacitive shields922-1, 922-2 may be connected to a common point (e.g., GND).

FIG. 9A shows a current sensor 902′ in accordance with some embodimentsof the present disclosure. The current sensor 902′ can be used to sensecurrent flowing in two conductors 542 a, 542 b. For example, the currentsensor 902′ may be used to sense current flow in the conductors of adifferential amplifier; see, for example, the configuration illustratedin FIG. 4B. The sense element 904′ may comprise first, second, and thirdcoils 912-1, 912-2, 912-3. The conductors 542 a, 542 b may haverespective segments 542 a-1, 542 b-1 that run on the substrate 932. Forexample, conductor segment 542 a-1 may run between coils 912-1 and912-2, and conductor segment 542 b-1 may run between 912-2 and 912-3.The current sensor 902′ may include capacitive shields 922-1, 922-2configured to shield the coils 912-1, 912-2 from an electric field thatcan emanate from conductor segment 542 a-1. The current sensor 902′ mayfurther include capacitive shields 922-3, 922-4 configured to shield thecoils 912-2, 912-3 from an electric field that can emanate fromconductor segment 542 b-1.

In accordance with the present disclosure, the single-conductor currentsensors (e.g., 402 in FIG. 5) may be used with a differential poweramplifier. Differential power amplifiers, for example, may be integratedin wireless power transmit circuitry to drive a transmit coil. FIGS. 10Aand 10B schematically depict illustrative embodiments of differentialpower amplifier configurations. FIG. 10A for example, shows adifferential power amplifier 1002 connected to loads 1004, 1006. Currentsensors 1000 a, 1000 b may be disposed along conductors 1042 a, 1042 bto sense a flow of current in the respective conductors. The currentsensors 1000 a, 1000 b may be connected together in series to produce asingle output (e.g., 408, FIG. 4B) that can be connected to an amplifier(e.g., 48, FIG. 4B). Referring to FIG. 6, for example, pad B of currentsensor 1000 a may be connected to pad A of current sensor 1000 b. Pad Aof current sensor 1000 a and pad B of current sensor 1000 b may be theinputs to an amplifier (e.g., 48).

FIG. 10B illustrates a configuration in which the conductors 1042 a,1042 b that are sensed by current sensors 1000 a, 1000 b may be disposedalong the ground paths from respective loads 1004, 1006. The currentsensors 1000 a, 1000 b may be connected in series. The configurationshown in FIG. 10B may be advantageous in some applications, since theline voltage in conductors 1042 a, 1042 b is close to ground potential.

FIG. 10C illustrates a configuration of a dual-conductor single currentsensor 1000 c, such as illustrated in FIG. 9A for example, for sensingthe current flow in conductors 1042 a, 1042 b of the differentialamplifier 1002. The configuration shown in FIG. 10C shows the conductors1042 a, 1042 b to be along the ground path. In other embodiments,however, the conductors 1042 a, 1042 b that are sensed by the currentsensor 1000 c may be at the outputs of the differential power amplifier1002.

In still other embodiments, three or more current sensors may be used.For example, the configuration two single-conductor current sensors 1000a, 1000 b shown in FIG. 10B may be combined in series fashion with thedual-conductor current sensor 1000 c shown in FIG. 10C. FIG. 10Dillustrates an example of such a configuration.

Current sensors may be used in wireless power circuitry; e.g., toprovide feedback for power control. Current sensors may be particularlyuseful for lost power determination. For example, current sensors mayused detect an amount of power transmitted in order to determine theamount of power lost based on what the receiver is receiving, or todetect the presence of objects consuming power on the pad.

Current sensors in accordance with the present disclosure do notinteract directly with the current flow that is being sensed. Therefore,the current sensor creates no imbalance in the power amplifier thatsupplies the current. In addition, current sensors in accordance withthe present disclosure can provide an output voltage that is isolatedfrom the output of the power amplifier.

Current sensors in accordance with the present disclosure do not useswitching circuitry, and so do not emit EMI that is typically associatedwith the use of switching circuitry.

Current sensors in accordance with the present disclosure can create avoltage waveform that is 90 degrees out of phase with current and thuscan provide a usable phase angle measurement of the current flow. Inaddition, the zero crossing of this waveform can be compared to that ofthe power amplifier output voltage to provide an accurate measure ofphase angle. This phase angle can be used for both load power andimpedance measurements.

The above description illustrates various embodiments of the presentdisclosure along with examples of how aspects of the particularembodiments may be implemented. The above examples should not be deemedto be the only embodiments, and are presented to illustrate theflexibility and advantages of the particular embodiments as defined bythe following claims. Based on the above disclosure and the followingclaims, other arrangements, embodiments, implementations and equivalentsmay be employed without departing from the scope of the presentdisclosure as defined by the claims.

What is claimed is:
 1. A current sensor comprising: a conductor; a senseelement configured to couple to a first magnetic field generated at afirst location due to a flow of current in the conductor; and an outputelectrically connected to the sense element and configured to produce asignal that is representative of the flow of current in the conductor,the sense element oriented in a plane parallel to magnetic field linesof a second magnetic field, different from the first magnetic field,generated by a load electrically connected to the conductor at a secondlocation different from the first location.
 2. The current sensor ofclaim 1, further comprising a substrate, wherein the sense elementcomprises an electrically conductive coil disposed on the substrate andadjacent the conductor.
 3. The current sensor of claim 2, furthercomprising a filter circuit, the filter circuit comprising theelectrically conductive coil.
 4. The current sensor of claim 3, whereinthe filter circuit is a bandpass filter.
 5. The current sensor of claim2, wherein the substrate is disposed perpendicular to a printed circuitboard (PCB) that carries the conductor.
 6. The current sensor of claim1, wherein the load comprises a power transmitting element configured togenerate an external magnetic field for wireless power transfer, whereinthe external magnetic field constitutes the second magnetic field. 7.The current sensor of claim 6, wherein the sense element comprises anelectrically conductive coil, the current sensor further comprising afilter comprising the electrically conductive coil of the sense element,wherein the filter is tuned to a resonant frequency of the powertransmitting element.
 8. The current sensor of claim 1, furthercomprising a capacitive shield disposed between the sense element andthe conductor, the capacitive shield effective to prevent an electricfield generated in the conductor from being capacitively coupled to thesense element.
 9. The current sensor of claim 8, wherein the capacitiveshield comprises a conductive lead disposed adjacent to both the senseelement and the conductor, wherein the conductive lead comprises a freefirst end and a second end configured to electrically connect to groundpotential.
 10. The current sensor of claim 1, wherein the sense elementcomprises a first electrically conductive coil disposed on a first planeand at least a second electrically conductive coil disposed on at leasta second plane spaced apart from the first plane.
 11. The current sensorof claim 10, further comprising a first capacitive shield configured tobe disposed adjacent to both the first electrically conductive coil andthe conductor, and at least a second capacitive shield configured to bedisposed adjacent to both the second electrically conductive coil andthe conductor.
 12. The current sensor of claim 10, wherein the firstelectrically conductive coil is connected in series with the secondelectrically conductive coil.
 13. The current sensor of claim 10,wherein the first electrically conductive coil comprises a first traceformed on a first layer of a multi-layer PCB and the second electricallyconductive coil comprises a second trace formed on a second layer of themulti-layer PCB.
 14. The current sensor of claim 10, wherein the firstelectrically conductive coil is disposed on one side of the conductorand the second electrically conductive coil is disposed on another sideof the conductor.
 15. The current sensor of claim 10, further comprisinga filter circuit, the filter circuit comprising either or both the firstelectrically conductive coil and the second electrically conductivecoil.
 16. The current sensor of claim 1, wherein the sense elementcomprises a first electrically conductive coil and at least a secondelectrically conductive coil, the first and second coils both spacedapart from each other on a same plane.
 17. A method of sensing a flow ofcurrent in a conductor comprising: generating a sensed signal bymagnetically coupling to a first magnetic field arising from theconductor due to the flow of current in the conductor, the magneticcoupling occurring in a plane parallel to field lines of a secondmagnetic field generated by the flow of current through a loadelectrically connected to the conductor; and generating an output signalfrom the sensed signal, the output signal representative of an amount ofthe flow of current through the conductor.
 18. The method of claim 17,wherein magnetically coupling to the first magnetic field includesinducing a current, using the first magnetic field, in a coil ofelectrically conductive material disposed adjacent the conductor andaligned in the plane parallel to field lines of the second magneticfield.
 19. The method of claim 18, further comprising filtering thesensed signal to attenuate frequency components in the sensed signaldetermined by a filter circuit that includes the coil of electricallyconductive material.
 20. The method of claim 18, wherein the loadcomprises a power transmitting element configured to generate anexternal magnetic field for wireless power transfer, wherein theexternal magnetic field constitutes the second magnetic field.
 21. Themethod of claim 17, further comprising shielding the sensed signal froman electric field generated in the conductor so that the generatedoutput signal is substantially free of influence from the electricfield.
 22. The method of claim 17, wherein magnetically coupling to thefirst magnetic field includes coupling the first magnetic field to afirst coil of electrically conductive material disposed adjacent theconductor and coupling the first magnetic field to a second coil ofelectrically conductive material disposed adjacent the conductor.
 23. Anapparatus for sensing a flow of current in a conductor comprising: meansfor magnetically coupling to a first magnetic field arising from theconductor due to the flow of current in the conductor to generate asensed signal, the means for magnetically coupling to a first magneticfield being aligned in a plane parallel to field lines of a secondmagnetic field generated by the flow of current through a loadelectrically connected to the conductor; and means for generating anoutput signal from the sensed signal.
 24. The apparatus of claim 23,further comprising means for filtering the sensed signal, the means forfiltering comprising the means for magnetically coupling to a firstmagnetic field.
 25. The apparatus of claim 24, wherein the means formagnetically coupling to a first magnetic field comprises a coil ofelectrically conductive material.
 26. The apparatus of claim 24, whereinthe means for filtering is a bandpass filter.
 27. An apparatus forwirelessly transmitting charging power to a receiver device, comprising:a transmit coil; a driver circuit electrically coupled to the transmitcoil via a conductor, the driver circuit configured to drive thetransmit coil with an alternating current via the conductor; and acurrent sensor configured to sense a flow of current in the conductor,the current sensor comprising a sense coil configured to couple to afirst magnetic field generated by the alternating current in theconductor to produce a signal that is indicative of the flow of currentin the conductor, the transmit coil configured to generate a secondmagnetic field for wirelessly transmitting charging power to thereceiver device in response to being driven by the alternating current,the sense coil oriented in a plane parallel to field lines of the secondmagnetic field.
 28. The apparatus of claim 27, further comprising afilter circuit, the filter circuit comprising the sense coil and aresistor and capacitor electrically connected to the sense coil.
 29. Theapparatus of claim 28, wherein the filter circuit is a bandpass filter.30. The apparatus of claim 27, wherein the current sensor furthercomprises a capacitive shield disposed adjacent to the sense coil, thecapacitive shield effective to prevent an electric field generated inthe conductor from being capacitively coupled to the sense coil, whereinthe capacitive shield comprises a second conductor disposed between theconductor and the sense coil.